In addition to Am1rr3za's answers (other than the CPU, which is soldered to the logic board), you can also convert the optical drive bay to a caddy which houses a second hard drive or solid state drive. This is a huge upgrade if you can stand being without an internal disc drive...

Not if you don't break anything, are careful not to strip screws or remove stickers, and put the optical drive back in before you take it back in for service :) Who really knows what actually voids an Apple warranty? They seem to evaluate it on a case by case basis (or based on what "genius" happens to be working that day). I say don't worry about it, be careful, and do what you want with the product you paid upwards of one thousand dollars for. If you bark loud enough later and haven't filled the case with vaseline, they'll honor your warranty.
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Robert S CiaccioSep 19 '10 at 2:54